Sorry, replied to the wrong person before, edited.
CaroleW - Yes, that's a strong possibility. The spouses had no previous marriages I'm aware of (both the post-1855 ones are listed as spinsters on the records) so she may well be illegitimate.
William married Helen Booth in 1843. She died in 1858.
He then married Margaret Innes in 1860, she died later that same year after giving birth to Isabella.
William married Margaret Henderson Ross in 1862.
The ages of most people in the family on the 1871 are correct to within 1 or 2 years, therefore Eliza was likely born around 1856-1860, after statutory registration came into force.
Have looked for Eliza/beth (& variants) with the surnames Forsyth, Booth, Innes, and Ross in the past (see end), though she could be registered under a different father's surname. A couple I did find I followed through time only to prove they weren't the right ones....
Have also expanded all three of the spouse families, looking at brothers and sisters for each, looking for any potentially matching nieces or nephews with them on the 1861 census, without luck.
The best looking birth match is an Elizabeth Ross (illegitimate), born 1860 to Margaret Ross, a weaver, living on Ann Street. The later censuses for Margaret H Ross show her working as a net weaver after William's death.
Edit: and now found her death, which confirms the above birth record is correct. Hadn't expected to get this solved when asking for a handwriting check!
